The present invention relates to methods of measuring the temperature of an electronic device. It is to be understood that the use of the term electronic is intended to encompass electronic, optoelectronic and photonic devices, semiconductor based or otherwise.
It is well known that the temperature of an active electronic or opto-electronic device has an effect on the device performance and reliability. At a basic level, this understanding has resulted in the widespread use of heat sinks to which the active devices are attached, the heat sinks including materials having good thermal conductivity and being designed to efficiently conduct heat away from the active device. Cooling fans and increased ventilation, as well as liquid coolants, have also been used to manage the temperature of active electronic devices.
However, it is becoming increasingly important to gain an understanding of the temperature variations actually within an active device to facilitate a greater understanding of how local areas of increased temperature can effect the device performance and to determine where within the active device such temperature build ups are likely to occur. This is becoming increasingly important as active device dimensions decrease, with the importance of thermal heating effects increasing in proportion to the decreasing device dimensions.
The most commonly used traditional method for non-invasive temperature measurement in active electronic devices is infra-red thermography, also known as infra-red thermal imaging. This technique has been used since the early 1960s and relies on the detection of the infra-red radiation emitted by a hot body. To make the thermal image measurement the entire chip containing the device to be measured must first be heated by contact with a temperature controlled heat sink to a temperature of typically 60-70xc2x0 C. The resulting infra-red emission from the device is then mapped and this measurement forms a calibration that is subsequently used to avoid interference from the background ambient temperature and to address local variations in surface emissivity. Depositing a layer, such as black paint on the device surface is sometimes used to eliminate surface emissivity variations. The device is then electrically operated and the thermal emission mapped again. The disadvantages of infra-red thermography include limited spatial resolution, dictated by the infra-red wavelength of approximately 3-12 xcexcm. This gives insufficient resolution for the most recently developed electronic devices having features of this size or smaller. In addition, the materials used to form some electronic devices have low surface temperature emissivity that can cause problems for accurate temperature mapping.
A more accurate alternative to infra-red thermography is the use of Raman scattering. This involves irradiating a target using a laser beam and measuring the light scattered from the target. At any given temperature there will be lattice vibrations within the crystalline structure of a semiconductor device. The laser light irradiated onto the device is inelastically scattered by the lattice vibrations. Inelastic scattering occurs when some energy is transferred during the scattering process. During Raman scattering either a phonon is created (Stokes-Raman scattering) or a phonon is annihilated (Anti-Stokes-Raman scattering). A phonon is the smallest energy quantum of the lattice vibrations. Therefore, because the laser light is inelastically scattered, the scattered light detected, by a spectrometer, has undergone a shift in energy equal to the phonon energy i.e. the frequency of the detected raman scattered laser light is equal to the frequency of the incident laser light plus or minus the frequency of a phonon. The frequency of the phonons varies as a characteristic of different materials, and more importantly varies for any given material as a function of temperature. Hence by detecting the frequency shift of scattered light from an electronic device it is possible to calculate the temperature of the device at the point of irradiation. Advantages of Raman scattering include the fact that no calibration on the measured device is necessary as each semiconductor material used in a device has its own characteristic temperature dependence of phonon frequency.
A further refinement of this technique is to use an optical microscope to focus the laser beam onto the sample and to collect the scattered laser light at a spectrometer. This is known as micro-Raman scattering. The spatial resolution achievable using micro-Raman scattering is directly dependent upon the size of the focused laser beam used to irradiate a sample and can thus be of the order of 1 xcexcm or better. This is directly comparable to the feature size of currently used semiconductor devices. Examples of methods and systems for measuring semiconductor device temperature using Raman scattering include French patent application FR 2772124 A and Japanese patent applications JP 11-337420 and JP 2001-085489.
However, the present applicants have identified a problem in using micro-Raman scattering associated with the ever decreasing device dimensions of the current and future electronic devices. As the individual features of current devices decrease in dimensions, examples of such features being individual metal contacts, the likelihood and necessity to measure the temperature of the device in close proximity to such a feature is increasing. It has been found that when a device is irradiated using a laser for micro-Raman scattering measurements in close proximity to a metal contact large shifts in phonon frequency are detected that are artefacts in the measurement process as opposed to being due to an intrinsic property of the material under test. The presence of such large frequency shifts due solely to the close proximity of the metal contacts renders its problematic to accurately determine the temperature of the device at that particular point using conventional micro-Raman measurement techniques.
According to a first aspect of the present invention there is provided a method of measuring the temperature of an electronic device utilising a spectrometer having a slit through which scattered laser light is collected, the method including minimising the width of the spectrometer slit, irradiating an arbitrary location on an electronic device with a laser with the device being inactive, determining the phonon frequency of the device at the arbitrary location using the spectrometer and storing the determined phonon frequency, irradiating a target location on a device with the laser with the device being active and determining the further phonon frequency of the device at the target location with the spectrometer, determining the difference between the stored phonon frequency and further phonon frequency, and determining the temperature of the active device at the target location as a function of the determined phonon frequency difference.
Preferably a plurality of target locations are irradiated with the electronic device being active and the difference determined between the stored phonon frequency and the phonon frequency at each of the plurality of target locations.
According to a second aspect of the present invention there is provided a method of determining the temperature of an electronic device by means of a laser spectrometer, the method comprising minimising the slit width of the spectrometer, measuring the temperature by Raman scattering at a first location on the device with the device inactive at a known ambient temperature, measuring the temperature by Raman scattering at a second location with the device active and summing the difference between said temperatures with the known ambient temperature.
According to a third aspect of the present invention there is provided a method of measuring the temperature of an electronic device, the method comprising irradiating a target location on the device using a laser with the device being inactive and determining a first phonon frequency of the device at the target location, irradiating the target location using the laser with the device being active and determining a second phonon frequency at the target location, determining the difference between the first and second phonon frequency, and determining the temperature of the active device at the target location as a function of the determined phonon frequency difference.
According to a further aspect of the present invention there is provided a method of determining the temperature of an electronic device, the method comprising measuring a first temperature of the device using Raman scattering at a target location with the device inactive, measuring a second temperature of the device using Raman scattering at the target location with the device active and summing the difference between the first and second temperatures to a known ambient temperature.
In the above methods, the electronic device preferably comprises a plurality of portions, each portion having a predetermined bandgap.
Preferably, the wavelength of the laser is selected such that it is less than the bandgap of at least one of the portions.
The laser may be focused on a selected portion using confocal microscopy or alternatively the determined phonon frequency of a selected portion is determined from within an expected frequency range, the expected frequency range being dependent on the material of the selected portion.
Preferably, the spectrometer comprises a charge coupled device and the binning width of the spectrometer is reduced so as to minimise a loss in spatial resolution.
The target location may be in sufficiently close proximity to a metal contact on the electronic device such that some of the laser irradiation is incident on the metal contact.
Additionally, the target position may be selected by detecting areas on the electronic device of comparatively high temperature by measuring infra-red radiation emitted by the device.
According to a further aspect of the present invention there is provided a method of measuring the stress within an electronic device at a target location on the device using a spectrometer having a slit through which scattered laser light is collected, the method including minimising the width of the spectrometer slit, irradiating an arbitrary unstressed location on the device with a laser, determining the phonon frequency of the device at said arbitrary location using the spectrometer and storing the determined phonon frequency, irradiating a target location on the device with the laser and determining the phonon frequency at the target location using the spectrometer, and determining an indication of the electronic device stress at the target location as a function of the difference in phonon frequency between the stored phonon frequency and the phonon frequency at the target location.